I. Field of the Invention
The present invention relates to solventless polymeric compositions and, more particularly, to generally liquid, low viscosity polymeric compositions which polymerize by means of an addition reaction mechanism.
II. The Prior Art
Many different types of liquid polymeric materials are known in the prior art and are used in numerous applications. For example, liquid polymeric compositions can be used for paints, coatings and films, or can be poured into molds or substrates to make castings. However, the various physical properties of these polymeric compositions present certain problems in many applications. For example, compositions having generally high viscosities or rapid reaction rates are difficult to apply as films or coatings without the use of solvents to dilute or dissolve the components. This normally creates pollution problems, however, when the solvent evaporates. Additionally, as the solvents evaporate they can leave voids or blisters in the polymeric material.
Accordingly, there exists a need in the art for solventless liquid polymeric compositions which can be used in paints and coatings, and which can be poured into molds to make castings. As used herein, the term "solventless" means the absence of any solvent in the components utilized to make the polymeric composition which does not take part in the reaction. Thus, upon mixing the various components, there is no solvent to be evaporated or entrapped into the resulting thermosetting polymeric materials. In other words, the whole system is 100% active and there is no waste through solvent evaporation. Under such solventless conditions, compact films or parts are formed which are void free, since there are no solvent losses through evaporation or solvent entrapments to cause porosity and blistering.
Furthermore, the absence of solvents eliminates environmental pollution caused by solvent loss. In this respect, solventless systems are even better than water borne polymeric materials because the latter contain small quantities of ammonia, amines and solvents used as water emulsifiers, solubilizers and stabilizers. Conventional solution polymeric materials used for coatings and paints contain substantial amounts of solvents or water which are lost by evaporation to the environment. Solventless systems have no evaporation problems and provide economy with their efficiency.
Based on the same weight and thickness of a coating, solventless polymeric systems provide considerably more surface coverage than solution systems because the solventless systems are 100% active. In addition, solventless systems are more compact with respect to production capacity, storage room, transportation room, and are safer, with less attendant health and fire hazards.
The best solventless compositions are thermoset through an addition polymerization mechanism. This simply means that component A reacts with component B by adding to it without the formation of any volatile by-products which could cause voids, blisters or entrapments in the film or part. These polymers can include ladder-like type polymers wherein the components are cross-linked spacially in the form of a ladder. For example, two linear parallel polymeric chains are intersected at their repeated reaction sites by components which form the steps of the spacial ladder.
The few ladder polymers which are known generally in the art have exceptionally good chemical resistance. However, these compositions are generally solids and are insoluble in organic solvents. Additionally, these ladder-type polymers generally have to be heated above 180.degree. C. to effect curing. Thus, it is extremely difficult to use a ladder-type composition as a paint or coating.
It would also be advantageous to have a solventless polymeric composition in which the various components are liquids with relatively low viscosities at ambient temperatures. Such systems would be advantageous because they could be conveniently applied as coatings by spraying, dipping, brushing, rolling, or the like.
Additionally, liquid, solventless compositions can be more easily mixed together with pigments, pigment extenders, dyes, fillers, carbon, glass or organic fibers without the necessity of dilution with solvents. Low viscosities greatly facilitate any grinding process and the absence of solvents reduces the fire hazard caused by the heat of friction in the presence of volatile and flammable organic solvents. Thus, the systems can readily be utilized to make paints and coatings.
Conventionally known polymeric materials used in coatings and paints have typically been of one or two component systems. The one component systems are generally always water borne and comprise solution or emulsion type materials in organic solvents such as oil alkyds, epoxy esters, urethane oils, and latexes. Two component systems generally include:
(a) solid or liquid epoxides and polyamines, or amine terminated polyamides; PA0 (b) isocyanate terminated polyurethanes and di or polyols, hydroxyl terminated polyurethanes and/or p,p'methylene,o,o', chloro dianiline; PA0 (c) unsaturated polyesters; and PA0 (d) photoprepolymers.
Presently available one component liquid polymeric systems have several distinct disadvantages. For example, the compositions can only be applied in thin coats so that the solvent can evaporate. Additionally, as the solvent evaporates, it can leave voids or blisters.
The two component systems also have certain disadvantages. For example, these systems which include epoxides have required a solvent to dissolve the epoxides such that they can be easily applied. Even the normally liquid diepoxides such as those based on Bisphenol A and epichlorohydrin (exemplified by EPON-826 and EPON-828 sold by Shell Chemical Company and others), have viscosities greater than about 4,000 centipoise (CPS) at ambient temperatures. Other liquid di or polyfunctional diepoxides of glycidyl ethers with Bisphenol A, as well as the liquid diepoxides of Novolac type, have viscosities much greater than 4,000 centipoise. When these liquid epoxides are cured with polyamines such as triethylene tetramine (TETA), or liquid amine-terminated polyamides such as the Versamides produced by General Mills Company, additional solvents must be used to reduce the viscosity. This is because when the epoxide and amines are mixed together, there is no viscosity decrease, but rather a rapid increase due to the high reactivity and functionality of TETA with the epoxides.
Both isocyanate and hydroxyl-terminated polyurethanes such as Adiprene L-100, Adiprene L-150 (produced by E. I. duPont de Nemours & Company of Wilmington, Del.), Multrathene-2680, Desmophens and Desmodurs (produced by Mobay Co.) have viscosities in excess of 18,000 centipoise or are solids at ambient temperature. Therefore, these compounds cannot be used as components of a liquid, solventless system.
Unsaturated polyesters such as the condensation product of maleic anhydride, phthalic anhydride or isophthalic acid with 15% to 25% molar excess of 1,2 propylene glycol diluted with 30% to 50% by weight of styrene monomer have been used as good, solventless systems when combined with 1% by weight of a ketone hydroperoxide catalyst and 0.3% by weight cobalt naphthoate as an accelerator. However, the volatility of the odorous styrene monomer, the highly critical 100/1/0.03 weight ratio and the very rigid and inflexible thin films which are produced make the styrene diluted unsaturated polyester system unsuitable for coatings or paints. However, these unsaturated polyesters are broadly used in combination with fiberglass to produce reinforced plastics.
Perhaps most similar to solventless systems are photoprepolymers composed of di or polyfunctional acrylates diluted with monofunctional acrylates containing polymerization inhibitors, photoinitiators (catalysts), and photoaccelerators (tertiary amines). However, these formulated prepolymers have limited storage life and have to be protected from contact with ultraviolet light. Additionally, the curing of these compositions requires strong sources of ultraviolet light or electron beam. These sources are generally accompanied by strong infrared light which causes evaporation of the unpleasantly smelling monoacrylic esters. Additionally, only films of a few mils in thickness can be fully cured. As soon as the surface of the photo prepolymer is exposed to ultraviolet light or electron beam it hardens, thus inhibiting further penetration of the ultraviolet light. This self inhibition problem of photoprepolymers is even more acute if the polymer contains pigments or fillers. Thus, photoprepolymers are generally limited to applications requiring very thin, and preferably clear, films.
In view of the foregoing, it is obvious that it would be a significant advancement in the art to provide a solventless polymeric composition which could be used for coatings and paints and also which could be poured into substrates or molds to form castings. It would be still a further advancement if such a solventless system had a liquid consistency of a relatively low viscosity at ambient temperatures.